Among many other different tasks, one basic function of elevator group control is the allocation of landing calls. The objective of allocation is to give calls to be served by the elevator cars in such a way that a performance indicator describing the elevator system is as good as possible. Conventionally the most commonly used performance indicators relate to call times and passenger waiting times. Typically averages are calculated from these times and their distributions are established.
There are many types of allocation methods for landing calls and each elevator manufacturer has its own methods for implementing this task. One common feature of all these different methods, however, is that they include a set of parameters specific to each method, with which the operation of the method used is affected.
The targets for monitoring are typically landing calls, car calls, the loads of the elevators and the states of motion of the elevators. At peak hours the aim can be giving priority to minimizing the travel time of a user of the elevator. Another common target for optimization, which is especially in the interests of the owner of the building, is the energy consumption of the elevator system.
Numerous targets for optimization can be found such as the call time, the estimated waiting time of the passengers, run time and travel time, the number of stops, the car load, the number of simultaneous car calls and landing calls, etc. What must be decided is which of these targets should be given priority and how much priority in which traffic situations.
Energy consumption is an important minimization target because the non-renewable energy resources on the planet are limited and growing energy consumption causes many indirect impacts e.g. in the form of the greenhouse effect. The operating expenses and maintenance expenses of buildings, for their part, can be influenced by using an elevator system that is economical in terms of its energy consumption. According to a study conducted in Hong Kong (Yim, Leung: “Building Towards Sustainability in Public Housing”, proceedings of the conference ‘Building for the 21st century’, London, 2001), of the energy consumption of the public spaces of one typical 40-storey residential building (i.e. excluding the personal electricity consumption used by the residents themselves) the elevator system consumes approx. 18 percent. Another study estimates that the elevator system uses between five and fifteen percent of the total energy consumption of a building. In order to reduce the energy consumption of one elevator, the transport capacity of the elevator, the motor, the ratios in the power input and generally the design of the mechanical parts of the system must be taken into account.
The international patent application WO 02/066356 presents a control method for an elevator system, in which the energy consumed by the elevator system is minimized such that the desired requirement of the service time of elevator passengers is fulfilled on average. In this method a target value is given for a certain service time of an elevator group and landing calls are allocated to different elevators such that over a longer time span the condition of the service time examined is fulfilled, but at the same time the energy consumption of the system is at its minimum. For example the call time from the giving of the call to the arrival of the elevator, the total travel time or the run time examining only the time spent in the elevator car can be used as a service time.
In one application according to patent application WO 02/066356 two magnitudes that are of different types and are non-commensurable are optimized, i.e. waiting time and energy consumption. For these magnitudes to be made commensurable and comparable with each other, the routes R of the elevators are selected such that the cost termC=WTTN(R)+WEEN(R)  (1)is minimized. TN(R) is the normalized sum of call times with route alternative R and likewise EN(R) is the normalized energy consumption caused by route alternative R. WT and WE are the weighting coefficients of the aforementioned cost terms such that0≦WT≦1 and WE=1−WT.  (2)
Publication U.S. Pat. No. 6,857,506 describes another type of call allocation method for an elevator system. In the method an energy consumption file is formed, which describes the energy consumptions of each possible elevator trip between two floors. The elements of the energy consumption file thus have the departure floor, the arrival floor and the load of the car as variables. When the energy consumed for each trip between two floors is known, the routing of the elevator cars can be calculated for the calls that are active such that the total energy consumption of the system is minimized.
Publication FI 115130 also relates to the controlling of an elevator group. In this method it is also possible to set a desired target value for a certain service time, such as for the average waiting time of passengers. In this case the aim is to minimize energy consumption such that an available model of the elevator system is utilized in the optimization. By means of the model the desired service time can be forecast. The system also includes a PID regulator that utilizes the forecast service times and thus the cost function can be optimized more effectively. Numerous route alternatives according to smaller energy consumption are obtained from the optimizer, from which the solution according to the target value of the desired service time is selected.
A common denominator for the above-described prior-art solutions is that the routes of the elevator cars are defined so that the change in potential energy of the system caused by transferring the passengers in the height direction is minimized. Considered in this context the system includes all the mass points that move in the vertical direction, in other words the elevator car with counterweight and the passengers of the elevators.
One problem with prior art is that in the minimizations of energy consumption according to prior art only the masses to be moved in the system and the length of journeys, i.e. the height difference between the departure floor and the arrival floor, have been considered. When considering the optimization of energy consumption it is possible to perform it more precisely by including the energy term relating to the speeds of the elevators.